Startle seizures are induced by sudden and unexpected stimuli. The startle (unexpected and sudden presentation of the stimulus) is the provoking factor, although, rarely, patients may be specifically sensitive to one sensory modality. Sudden noise is the main triggering stimulus, but somatosensory and less often visual stimuli are also effective in some patients. Habituation to repetitive stimulation occurs.

The startle response is brief (<30 sec); axial tonic posturing frequently causes falls, which are often traumatic. It is asymmetrical in 1/4 of patients. In hemiparetic patients, the seizure starts with flexion and abduction of the paretic arm and extension of the ipsilateral leg, which rapidly involve the contralateral side. Marked autonomic manifestations, automatisms, laughter, and jerks may occur concurrently. Atonic or myoclonic startle seizures are less common.

Seizures are frequent, occurring many times a day, and sometimes progress to status epilepticus.

Infrequent spontaneous seizures are common (probably in all patients).

Etiology
Various localized or diffuse static brain pathology, mainly due to pre-, perinatal or early insults. Common in Down syndrome.

Ictal EEG
A vertex discharge followed by diffuse flattening or low-voltage rhythmic (10 Hz) activity, which begins in lesioned motor or pre-motor cortex and spreads to mainly frontal regions. This is often obscured by muscle artefacts.

Differential diagnosis
(1) hyperekplexia (also called 'startle disease'), a non-epileptic disorder, and (2) reflex seizures (by touch, tap, or sudden dousing with hot water) may have a startle component that is not essential for their provocation; reflex seizures are mainly myoclonic, ictal EEG shows generalized discharges, and patients are otherwise normal.

Prognosis
Often poor.

Management options*
Treatment is often unsatisfactory with clonazepam, clobazam#, carbamazepine, or lamotrigine.